: From previous work on this project, the investigators determined calcium retention and kinetics in black and white adolescent girls. They developed a nonlinear regression model of the relationship between calcium intake and calcium retention in order to establish calcium requirements for maximal skeletal accretion. They now propose to study these same relationships in boys, hypothesizing that both calcium intake and stage of sexual maturity (associate with change in endocrine factors) will explain much of the variance in calcium accretion in boys as it did in girls. By controlling calcium intake in a metabolic study, the non-dietary factors, which drive calcium accretion, can be better understood. Boys are a better model than girls for studying endocrine factors which drive calcium accretion because of the fluctuation associated with menses in girls. The first aim establishes calcium requirements in boys. Peak velocity of calcium accretion determined by bone mineral densitometry in boys is 75 mg/d higher in boys than girls; thus calcium requirements may be higher for boys than girls. The second aim is to identify candidate hormones/growth factors which drive non-dietary changes in calcium. IGF-1 may be most predictive of initial gains in calcium accretion until attainment of peak velocity of calcium accretion in puberty and increases in sex steroid hormone levels may predict the subsequent decline in rate of calcium accretion. Calcium retention in boys will be determined over a range of calcium intakes between 700-2100 mg/d during three-week metabolic camps. Stable isotopes of calcium will be administered orally and by intravenous injection after the first week equilibrium period in a subset of boys. Analysis of complete urine and fecal collections and periodic blood samples will provide data for multicompartmental and stochastic analysis of calcium metabolism. Bone mass and total body and skeletal site calcium measured by dual energy X-ray absorptiometry, hormone levels (PTH, vitamin D metabolites, IGF-1, IGF-BP3, estrogen, testosterone, and SHBG), and biochemical markers of bone turnover (serum osteocalcin and bone specific alkaline phosphatase and urinary collagen crosslinks) will be determined in all boys. The relationship of calcium retention and bone turnover to parameters of sexual maturity and size will be analyzed. Optimizing calcium accretion during this rapid period of growth should maximize peak bone mass within the genetic potential and reduce risk of osteoporosis later.